Something wrong with C-AF on the 45mm 1.8?

I've noticed an issue shooting videos with my 45mm when set on C-AF, the camera tries to refocus every couple of seconds even when the scene is almost motionless, which results in video captures which are near unusable.

My 25mm 1.4 works fine when set on C-AF, is this an issue with lens construction or just my copy?

Camera body is an E-M5